Yes, a Y adapter is what you want, assuming the mechanical installation is appropriate (typically servos in each wing half in mirror position, or each pushrod connected to servo arm on opposite side from other servo - meaning that when both servos rotate the same direction, ailerons will move opposite each other). Otherwise, you'll need a reversing Y adapter (which reverses the signal on one of the two connectors). An elevon mixer won't work if your transmitter is only 4 channels. Even if the receiver has more, it won't get any signal from the transmitter for the other channels.